Friday, January 28, 2011

Review: Giant Leap Avionics Bay (4")

This avionics bay is made to be used with Giant Leap’s 4-inch flexible phenolic tubing. It also fits in LOC’s 4” tubing, but you should check the detailed dimensions on Giant Leap’s web site before planning to use it with anyone else’s tubes. It features dual threaded rods and an O-Ring seal on one end. I added ejection cannons, electrical connectors, audio plugs to break the ejection charge circuits, and a mounting board for my G-Wiz altimeter.



Construction:
Components provided by Giant Leap:

* One 7” phenolic coupler
* One bulkhead that is the same diameter as the coupler
* One bulkhead that fits inside the coupler
* One centering ring that fits inside the coupler
* One O-ring (ring OD = coupler ID)
* Two ¾” eyebolts, with washers and nuts
* Two pieces of ¼” all-thread
* Two sets of nuts and washers for the all-thread (for one end)
* Two wing nuts and washers (for the other end)

Components I added:

* One 2” section of Giant Leap 4” tube
* Two ¾” copper caps
* Two ¾” copper tubes ~2 ½” long
* Two banana plug jacks with screw terminals
* Two ¼” phono jacks, with plugs
* Four 6.75” sections of 1” x ¼” basswood
* Eight T-nuts and eight hex-head bolts
* Plywood to mount the G-Wiz
* Misc. nuts and bolts
* Misc. wire

Thee kit comes with two pages of illustrated instructions that show you how to assemble the unit, mount your electronics, and use it in your rocket. The intended use is to mount it between the drogue and main tubes in a dual-deployment rocket. The electronics bay basically consists of a piece of coupler tubing with two bulkheads that are clamped on either end by sections of all-thread. There is an eyebolt on both ends for attachment of the recovery system. All the required holes are pre-drilled.

One bulkhead sits on the outside of the coupler. The all thread is attached to this bulkhead using bolts and washers on both the inside and outside. Giant Leap recommends that these connections all be epoxied in place. On the opposite end, you epoxy the centering ring just inside the coupler. The O-ring sits against this ring and the second bulkhead, which fits inside the coupler, seats against the O-ring. Once clamped down, this provides a gas-tight seal. The all-thread fits through this second ring and wing nuts are used to hold it all together.

The drawbacks I saw were: 1) the tube is only positively sealed on one end (however, it appears to me that the other end fits tight enough to be essentially gas-tight); 2) there is no provision for routing wires through the bulkhead; and 3) there is no place to mount/hold the ejection canisters.

Since I wanted some method of externally breaking the path to the igniters, I added a 2” piece of tubing at the center of the coupler. Mounting holes for the phono jacks were drilled in the coupler and outer tube. The latter was drilled to fit their mounting nuts (the jacks are flush with the outer tube). I drilled three holes, even though only two jacks are needed (to help with pressure equalization).

I bought this avionics bay to use in my Landshark, which is a 4” dual-deployment rocket that uses ‘zipperless’ separation. I needed to attach the bay to both the drogue and main tubes, and to make sure it stayed put. I first marked and drilled four holes on each end of the coupler. I then marked the basswood slats to match, inserted T-nuts, and epoxied them inside the coupler tube. The hex-head bolts were inserted to keep the slats aligned. I had to be very careful not to get epoxy on the T-nuts!

The ejection cannons consist of a copper cap, which is bolted to each bulkhead. Another hole was drilled on the side of the cap to accept the leads form an electric match. A small piece of copper tube is press-fit into the cap.



For external electric connections, I chose screw terminal banana jacks. In addition to the banana jack (which is unused), these offer a way to tightly clamp a wire that is inserted through a hole in the terminal. I drilled small holes in the bulkheads to accept the jack’s terminals, which extend below its body. The jack is secured to the bulkhead with provided nuts and washers, and the wiring is attached via ring lugs.



For an electronics mount, I just cut a piece of thin plywood to fit across the all-thread. Between the all-thread, basswood strips, and wiring, the mounting plate is held in place well (i.e. it doesn’t bounce around inside). The wiring for the electric matches runs from the banana plug, through the phono jacks (one leg only) and to the G-Wiz. Externally, the ejection charge is inserted into the cannon with its leads running out the side, and is connected to the plugs.

Flight:
I have flown this electronics bay twice, and it worked great both times. I used the old-style ejection canisters from Pratt Hobbies. These consist of an electric match embedded in a small plastic vial and fit perfectly into the ejection cannons.

Summary:
This is a basic altimeter bay, which provides a gas-tight housing for your electronics. It would be easy to build a similar bay from scratch, but this way you get many of the piece parts in one kit. However, the user will have to make some modifications to make it fully functional. Sure, the electric matches can be routed through a small hole, the ejection charges could be taped to the plate, and no one makes you break the leads to the charges, but most people will want to do a bit more.

Wire routing is the main issue with my enhanced design. It takes some patience to get the mounting plate and wiring all nicely tucked into the bay. Other than that, it is easy to use and works well.